A fascinating “arms race” in the evolution of reproductive organs in male and female ducks has recently been uncovered by scientists. This research dispels the notion that female organisms are always passive participants in the intersexual relationship.
Evolving for Competitive Advantage
Up to 97% of bird species lack organs that function similarly to the human penis. Ducks belong to the rare 3% that possess a penis to aid in reproduction.
In bird species without a penis, mating occurs in a manner that biologists refer to as a “cloacal kiss”, which is a brief contact through an opening found in both males and females—this opening serves both excretory and reproductive functions.
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The reproductive organs of male and female ducks have evolved to counteract each other in a form of “arms race” for reproductive control. (Photo: Richard Seaman) |
Unlike many other bird species, male ducks exhibit a wide range of penis lengths, depending on the species, from a mere 1.27 cm to an impressive 38 cm! Furthermore, their penises also vary greatly in appearance, from smooth to spiky and ridged.
Scientists explain that male duck penises have evolved to be longer in order to gain a competitive advantage in delivering sperm to female duck eggs more quickly and with a higher likelihood of fertilization.
Patricia Brennan, a behavioral ecologist from Yale University in the United States and the University of Sheffield in the UK, stated: “After studying male duck genitalia, I was immediately fascinated by the thought of how the female reproductive structure might be designed to accommodate such an unusual male penis.”
Brennan and her colleagues were surprised to find that the female duck’s vagina is just as “elaborate” as the male’s penis. Their vaginas contain numerous “dead ends” and other countermeasures, seemingly designed to exclude any unwanted penis during mating.
Like a Lock and Key
In most bird species, the vagina or oviduct is a simple tube. However, in certain duck species, the vagina has pouches located on either side of its walls. These pouches serve as “dead ends” or “false entrances” for the penis.
Brennan explains: “If the penis enters these pouches, it gets blocked and cannot access the oviduct to deliver sperm.”
The oviduct of female ducks is also filled with tightly coiled structures. On this, ornithologist Richard Prum, Brennan’s collaborator, noted: “It’s interesting because the male duck penis is also spiral-shaped but in the opposite direction, which is counter-clockwise. Therefore, the vagina and penis in ducks act like a lock and key.”
The research team studied 14 different species of ducks and geese. Results indicated that over evolutionary time, the number of pouches and coiled structures in female reproductive organs increased, while male duck penises also grew longer.
Brennan remarked: “I was able to predict how the structures of male genitalia would need to fit with the female reproductive system, and vice versa.”
This study illustrates that the reproductive organs of both male and female ducks have evolved to counteract each other in a form of “arms race” for reproductive control.
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Depending on the species, the length of male duck penises varies widely, from a short 1.27 cm to a remarkable 38 cm! (Photo: LiveScience) |
Prum stated: “Although most duck species are monogamous, forced copulation by males is a common occurrence in many duck species. The length of the penis in a given species is strongly related to the frequency of forced copulation in that species.”
Therefore, according to Brennan: “In response to male ‘rape’ attempts, females have the ability to use their own physical and behavioral conditions to determine which male gets to ‘father’.”
This means that male duck penises have evolved to overcome the increasingly sophisticated defenses of female duck reproductive organs, and conversely, female reproductive systems have evolved even further to fend off male “attacks.”
She noted: “Some larger, monogamous bird species, like geese and swans, have smaller penises, while others that are smaller and promiscuous have larger and more complex reproductive organs.”
The Active Choice Mechanism of the Female Organism
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The evolutionary case seen in ducks is part of the interactions in the reproductive processes of all animal species, including humans. (Photo: Richard Seaman) |
According to the research team, when a female duck is receptive to a male’s “courtship,” she will show cooperative behavior by allowing the male’s penis to bypass the barricade she has created. If she is determined to reject the male, she will prevent the penis from escaping the “dead ends” within her vagina.
Robert Montgomerie, an evolutionary biologist at Queen’s University in Kingston, Canada, commented: “These findings eliminate the notion that female organisms are always passive participants in intersexual relationships.”
He believes that this research serves as a reminder that attention must be equally given to both sexes in the reproductive process. Meanwhile, ornithologist Kevin Johnson suggested expanding the research to other animal species to discover similar evolutionary cases.
In discussing her research, Brennan stated: “What truly fascinates me is the mysterious selection mechanism of females. Imagine an evolutionary environment in which males have longer penises while females create more barriers in their reproductive systems for self-protection. This is indeed an evolutionary ‘arms race.’
“We believe that such evolutionary cases are widespread and are part of the reproductive interactions of all animal species, including humans.”
This study was published in the journal PloS One (Public Library of Science) on May 2, 2007.
Minh Quang
